In a series of investigations currently being published under the general title, "Studies on Resistance in Snails:" (7 papers published, 3 in press), the research team of Drs. Lie, Heyneman and collaborators has uncovered evidence of a specific immological system operating in the snail hosts of human schistosomiasis and other trematode infections. This is the first demonstration in an invertebrate system of a nanmnestic specific response system with a hemapoietic center. Several types of encapsulation response to specific stimuli have been described, using irradiated miracidia, which elicit in the host a striking capacity to encapsulate and then become specifically sensitized to the homologous parasite. Both innate (genetic) and experimentally acquired immunity can be reinforced or established by this procedure. Particularly intriguing is the fact that natural or acquired immunity can be temporarily or permanently broken under certain experimental conditions. These multiple infection experimental approaches, a variety of genetically selected snail strains of Biomphalaria glabrata, and several strains of the human schistosome fluke, Schistosoma mansoni, as well as five species of echinostomes isolated in the laboratory are now available for these interaction studies. We now have a research tool or an infection model that provide new information of both theoretic and applied importance in host-parasite interactions and invertebrate immunology. BIBLIOGRAPHIC REFERENCES: Lie, K.J. and Heyneman, D. (1976). Studies on resistance in snails. 3. Tissue reactions to Echinostoma lindoense sporocysts in sensitzed and resensitized Biomphalaria glabrata. J. Parasit. 62(1):51-58. Lie, K.J., Heynmen, d., and Jeong, KH (1976). Studies on resistance in snails. 4. Induction of ventricular capsules and changes in the amebocyte-producing organ during sensitization of Biomphalaria glabrata snails. J. Parasit. 62(2):286-291.